This invention relates to an on-load tap changer for use with an electric apparatus such as a transformer, and more particularly, to a compact on-load tap changer of a selector switch type, comprising a selector switch and a change-over switch enclosed within an insulating housing.
A conventional on-load tap changer of a selector switch type is schematically shown in FIGS. 1, 5, 6, 7 and 8. In FIG. 1, an on-load tap changer 10 is shown consisting of an electrically insulating housing 12, a selector switch 24 and a change-over switch 44. The electrically insulating housing 12 comprises a first insulating housing 14, a second insulating housing 16 and partition wall 18 between the two housings 14 and 16. The interior of the housing 12 is divided by the partition wall 18 into a first compartment 20 and a second compartment 22.
The selector switch 24 is installed in the first compartment 20 isolated from the transformer insulating oil by the first insulating housing 14 and the partition wall 18, and consists of an output shaft 31 operated by a quick motion mechanism 28 with a worm wheel 26, movable contacts 32 with a rotary contact system and fixed contacts 34. The movable contacts 32 of the selector switch 24 are mounted on the output shaft 31, and the fixed contacts 34 of the selector switch 24 on the inner surface of the first insulating housing 14. The contacts 32 and 34 of the selector switch 24 are divided into three groups each for the respective phases and the contact groups are axially stacked in the direction of the axis of the tap changer 10. When the worm wheel 26 is rotated, a rotating shaft 30 connected at one end (the upper end in FIG. 1) to the worm wheel 26 is rotated and a desired energy is accumulated in the tension spring within the quick motion mechanism 28. After this, the energy accumulated in the tension spring is released accordingly to rotate the output shaft 31 and select a desired tap position.
The other end (the lower end in FIG. 1) of the rotating shaft 30 is connected to one end of an input shaft 36 through a coupling 38. The input shaft 36 is rotatably supported by a bearing 40 carried by the partition wall 18 and extends at the other end through the partition wall 18 into the second compartment 22 within the second insulating housing 16. The other or lower end of the input shaft 36 is connected to an intermittent drive mechanism 42 disposed within the second compartment 22 immersed in the transformer oil. A change-over switch 44 is also disposed within the second compartment 22 and is connected to the intermittent drive mechanism 42 through an output shaft 46. The change-over switch 44 comprises a plurality of movable roller contacts 48 secured to the output shaft 46 through a contact holder and a plurality of fixed contacts 50 mounted on the inner surface of the second insulating housing 16. When the output shaft 46 rotates, the movable contacts 48 rotate relative to the fixed contacts 50 to effect switching according to the rotational position of the movable contacts 48.
In the conventional on-load tap changer described above, the number of the fixed contacts 32 of the selector switch 24 is very large. Therefore, when this large number of contacts 32 are to be disposed circumferentially within the first insulating housing 14, they must be phase-divided in the direction of the axis of the first insulating housing 14 into first, second and third phases, for example Phase I, Phase II and Phase III in FIG. 1). The fixed contacts in each phase include respective collector contacts 39 at the bottom ends thereof as shown in FIG. 1. Thus, the axial length of the entire on-load tap changer 10 is inevitably increased and the tap changer 10 becomes large-sized. Therefore, the only way of reducing the entire length of the on-load tap changer 10 and making the device small-sized is to reduce the axial length of the change-over switch 44 in the second insulating housing 16.
This problem of increased length of the tap changer is particularly serious with an on-load tap changer for use with an electrical transformer using a delta connection as shown in FIG. 2.
That is, in FIG. 2 in which three transformer windings are connected in the delta connection, each of the transformer windings comprises a first and a second transformer main winding 52 and 54, and a tap winding 56 between the transformer main windings 52 and 54. The first main winding 52 and the tap winding 56 are connected by the change-over switch 44, and the tap winding 56 and the second main winding 54 are connected by the selector switch 24. The first transformer main winding 52 has taps on the fixed contacts 50 that can be selectively connected by the movable contacts 48 (FIG. 1) of the change-over switch 44. The taps with which the change-over switch 44 can be connected provide a change-over switch winding 58 which is a section of the transformer main winding 52. The tap winding 56 has a plurality of tappings which are connected to the fixed contacts 34 (FIG. 1) of the selector switch 24 and which can be selectively connected by the movable contacts 32 (FIG. 1) of the tap changer 24.
Although not illustrated, three change-over switches 44 and three selector switches 24 of three phases are mechanically linked so that they are actuated in unison. The voltage at the tap winding 56 is generally 10% of the phase voltage, and the change-over switch winding 58 of the transformer main winding 52 has a voltage substantially identical to that of the tap winding 56. As apparent from FIG. 2, about one half of the interline voltage of the transformer main windings 52 and 54 appears between the phases between the selector switch 24 and the change-over switch 44, and as described above, the inter-phase distance of the selector switch 24 which is phase-divided in the axial direction is inevitably axially elongated as compared with the case where the connection used is a star connection.